As well as claiming that the Croat striker remains part of Pep Guardiola's plans, the Bayern chief rejected reports linking them with David Luiz
The striker, 27, who has scored of 10 Bundesliga goals this season, has been linked with a move away from the Allianz Arena in the summer, coinciding with Lewandowski's arrival from Borussia Dortmund on a free transfer.
However, Rummenigge has moved quickly to quash such speculation following confirmation of the contract with the Polish attacker, insisting that the Bavarians are in fact hoping to extend the Croat's current deal.
"We're very pleased with Mario," Rummenigge told Bayern's official website. "He made a significant contribution to us winning so many trophies last year.
"He's a great player who is thriving at Bayern. He has a contract with us until 30 June 2016, and I told his agent during the Club World Cup that we're willing to consider an early contract extension.
"It is by no means a topic of discussion that we want to sell Mario."
Chelsea defender David Luiz has been heavily linked with Pep Guardiola's squad, but Rummenigge claims that the reported interest has been invented as die Roten are not looking at bringing in the likes of the Brazil international this month.
"There's a lot of speculation at the moment, especially in England," the Bayern chief added. "We're being linked with David Luiz and a number of Spanish players.
"All I can say, very clearly, is bringing in these players is not of interest to us. We're not concerned about players we're being linked with joining next summer.
"There's nothing lined up, so we have nothing to hide. It's simply not an issue."
Bayern return to Bundesliga action, having lifted the Club Word Cup in December, on January 25 at Borussia Monchengladbach.